1. Field of the Invention
The present invention relates to lasers and particularly to a flashpumped solid state laser which operates at a wavelength of 2.09 microns at or near room temperature and with a high slope efficiency of at least 4%.
2. Description of the Prior Art
Development of room temperature solid state lasers in the two micron spectral range has received renewed attention recently because of potential applications in medicine and optical communications.
Demonstrations of laser action around 2.0 microns, using the .sup.5 I.sub.7 -.sup.5 I.sub.8 transition of Ho.sup.3+, as well as the .sup.3 F.sub.4 -.sup.3 H.sub.6 transition of Tm.sup.3+, appeared among the early reports on rare earth ion lasers. One difficulty with both of these 2.0 micron transitions is due to the fact that the terminal laser level is only separated from the ground state by a Stark splitting on the order of 10.sup.2 -10.sup.3 cm.sup.-1. Thus, the early laser demonstrations were performed at cryogenic temperatures in order to reduce the Boltzmann population of the lower laser level.
Early work on room temperature, 2 microm, solid state lasers has been conducted on a combination using Cr.sup.3+ and Tm.sup.3+ as co-dopant sensitizers and Ho.sup.3+ as a dopant activator. In such early work on this sensitizer/activator combination, a lasing or slope efficiency of 0.5% was reported, without optimization of the host material, ion concentration or design parameters of the external laser system that was used. Operation of such a laser with this low slope efficiency of 0.5% would produce an extremely low output laser power level under normal operating conditions. Such a resultant low output laser power level would limit such a laser a very low power applications. In order to achieve a reasonable amount of output laser power from such a laser, a large and expensive, associated input power supply would be necessary.
Subsequent efforts have been undertaken to optimize several of the material and system parameters in order to increase laser slope efficiency. Many of the recent studies on Cr.sup.3+ -sensitized 2 micron lasers have concentrated on the scandium gallium garnets, YSGG and GSGG, as host materials. One such study used the above-mentioned Cr:Tm:Ho: dopant system in a YSGG host material to demonstrate a slope efficiency of 3.1% in a single, flashlamp-pumped, room temperature, 2 micron laser. This choice of host crystal material appears to have been based primarily on the high efficiency of Cr:Nd lasers in the scandium gallium garnets. Superior laser performance of Cr:Nd:YSGG (or GSGG) is attributed mainly to higher Cr.sup.3+ cross sections and higher Cr to Nd transfer efficiencies in those hosts in comparison with YAG. However, in order to achieve these high transfer efficiencies, the scandium gallium garnet laser materials were required to contain very high concentrations of Cr.sup.3+.
Such prior art work failed to utilized optimum concentrations of dopants and the best host material for a Cr:Tm:Ho: laser. In addition, the prior art work failed to realize that, in order to select the best host material for a Cr:Tm:Ho laser, it is necessary to determine the Cr to Tm energy transfer efficiency as a function of host crystal and dopant concentration. Thus, there is a long-standing need for improving the slope efficiency of a garnet crystal laser operating at room temperatures by utilizing optimum concentrations of dopants in the host material of the garnet crystal laser.